During airport approach, when the engines of an aircraft are near idle condition and when the high-lift systems and landing gears are deployed, airframe noise is the dominant noise source. The noise that is generated at the side edge of the flaps has been identified as an important airframe noise component and is a target for noise control.
A number of numerical and experimental studies have been conducted in order to identify and model the noise generation mechanisms at the flap side edge. Flow field measurements in the flap side edge region of a wing with a half-span flap have revealed the presence of a 2-vortex system: a small vortex near the flap side edge on the top surface and a stronger side vortex along the lower portion of the flap side edge. As it travels downstream along the flap side edge, the side vortex strengthens and expands. At about mid-chord, it begins to spill over the flap top surface and merges with the small top vortex. The instabilities in this vortex system and in the strong shear layer that originates on the bottom edge of the flap create an unsteady pressure field at the flap side edge causing sound to radiate. It has been determined in the known art (Brooks, T. F. and Humphreys, W. M. Jr.: Flap Edge Aeroacoustic Measurements and Predictions. AIAA 2000-1975), that the dominant flap side edge noise emission region are located around mid-chord on the pressure side of the flap edge and around 60-65% chord on the suction side. These noise emission regions are aft of the front leading edge region where the vortex initially forms—from the leading edge at 0% chord to about 35% of the chord.
Some noise reduction concepts have been evaluated in the known art. For example, in Koop et al., Reduction of Flap Side Edge Noise by Active Flow Control, AIAA 2002-2469, some noise reduction was achieved by blowing air into the flap side edge vortex. The air was blown through a series of small round orifices located along the top and bottom side edges of the flap between 13 and 35% chord—where the vortex forms.